1. Field of the Invention
This invention relates to optical communications and, in particular, to the procedures employed in the manufacture of glass optical fibers.
2. Art Background
In the manufacture of optical fibers, a glass preform, which is a selectively doped glass tube, is fabricated by a process such as MCVD (modified chemical vapor deposition), MCVD performed with a plasma, or PCVD described by D. Kuppers et al, in the Journal of the Electrochemical Society, 423, 1079 (1976). The preform is either constricted or sealed on one end, collapsed into a solid body, and simultaneously with the collapse or subsequently after the collapse, an optical fiber is drawn from the solid body. Bound OH moieties in the preform and in the resulting fiber absorb in the wavelength region typically employed in optical communication systems and substantially increase the signal loss in such systems. Thus, during the formation of the preform great care is taken to substantially exclude the incorporation of OH moieties, e.g., SiOH, into the preform. Generally, the predominant source of OH moieties involves hydrogen-containing entities which at the collapse temperature are typically converted to water. The water, in turn, reacts with the preform to produce bound OH moieties. Thus, to maintain the quality of the fiber ultimately produced, substantial measures are also taken during preform collapse to exclude hydrogen-containing entities.
One predominant method has been employed to prevent the incorporation of OH moieties during preform collapse. In this procedure described by K. L. Walker et al, "Reduction of Hydroxyl Contamination In Optical Fiber Preforms," Third International Conference on Integrated Optics and Optical Fiber Communications, San Francisco, Calif., Apr. 27-29, 1981, (New York: IEEE, 1981), WA4, 86-88 (1981), molecular chlorine is introduced during the collapse procedure. The chlorine generally reacts with water, e.g., water formed from hydrogen-containing entities, to produce hydrogen chloride through the reaction EQU H.sub.2 O+Cl.sub.2 .revreaction.2HCl+1/20.sub.2. (1)
The resulting HCl is not incorporated into the preform and is removed in the effluent. This molecular chlorine collapse procedure has been found to produce fibers with relatively low losses due to OH absorption--losses generally in the range 0.3 to 2 dB/km at the OH absorption peak wavelength of 1.39 .mu.m. Nevertheless, it is certainly advantageous despite the acceptable level of loss already achieved, to substantially decrease loss to even more desirable levels.